Cardiac muscle

Question 1

Where is cardiac muscle found?

Answer 1

In the heart wall and in the base of large veins

Question 2

What are the functions of the cardiac muscle?

Answer 2

Needs to contract forcefully and rhythmically, modifying according to circulatory needs

Question 3

What is the structure of cardiac muscle?

Answer 3

• Formed from cardiomyoctes which consist of myofibrils and sarcomeres (striated), are small with single nuclei and packed with mitochondria
• Spirally arranged in a branching linear array
• Do not divide!

Question 4

What are intercalated disks?

Answer 4

• Join individual cells
• Formed from demosomes (mechanically hold cells together) and gap junctions (allow electrical and chemical communication)

Question 5

How do the mechanisms of an action potential and the release of Ca2+ differ in cardiac muscle from skeletal muscle?

Answer 5

Action potential requires Na+ and Ca2+

Ca2+ release from SR through RyR is triggered by Ca2+ inflow rather than coupling with DHPR

Question 6

What are the two specialised types of cardiac muscle cells?

Answer 6

1) contractile cells

2) pacemaker cells

Question 7

How do pacemaker cells maintain the heart rhythm?

Answer 7

Show regular spikes with slow depolarisation before spikes

Question 8

What mediates the rising phase of pacemaker cells?

Answer 8

• Driven by inflow of Ca2+ via v dependent channels

Question 9

What mediates the falling phase of pacemaker cells?

Answer 9

Driven by K+ output by v dependent channels

Question 10

What is the ‘funny current’ mediated by?

Answer 10

Slow inflow of Na+ which is activated by hyperpolarisation and inactivated by depolarisation

Question 11

What are the 4 sites at which pacemaker cells occur?

Answer 11

• sinoatrial node
• atrioventricular node
• bundle of His
• Purkinje fibres

Question 12

In what sequence does excitation pass through the heart via the pacemaker cells?

Answer 12

• SA node (major pacemaker)
• AP spreads through gap junctions in atrium
• Delay in AV node so atria contract first
• AP then spreads through the ventricles

Question 13

How are the cardiac membrane systems different to that of skeletal muscle?

Answer 13

• Larger T-tubules
• SR less well organised with a diad
• DHPR is v dependent Ca2+ and not directly linked to RyR2 which instead is Ca2+ activated
• Small Ca2+ entering through DHPR causes large release of Ca2+ by RyR2 through the SR

Question 14

What are the two forms of regulation on cardiac muscle?

Answer 14

ionotropic: force
chronotropic: rate

Question 15

What systems exert change on the cardiac muscle?

Answer 15

Nervous and endocrine system:

• Parasympathetic (Ach)
• Sympathetic (noradrenaline)

Question 16

How is neural control exerted on the heart?

Answer 16

2 paired control centres in the medulla oblongata: cardio accelerator centre (sympathetic) and cardioinhibitory centre (parasympathetic activity through the vagus nerve)

Question 17

How does sympathetic stimulation reach the cardiac muscle?

Answer 17

• Increase in AC causes increase in cAMP and protein kinase which causes more phosphorylation
• This enhances the funny current causing more Na+ to enter speeding pacemaker depolarisation (chronotropy)
• Also enhances Ca2+ current causing stronger contraction (ionotropy)

Question 18

How does parasympathetic regulation reach the cardiac muscle?

Answer 18

• Ach activates m2AchR working through a second messenger cascade
• Inhibits AC and reduced cAMP
• Down regulation of Ca2+ v dependent channels and up regulates K+ channel causing more negative pacemaker after a spike

Question 19

Describe the pattern of an action potential in the cardiomyocytes

Answer 19

1 - rapid depolarisation due to Na+ channels
2 - Small repolarisation due to transient K+ channels
3 - Plateau as L-type Ca2+ channels stay on for a long time (turned on by Na+ depolarisation)
4- Rapid repolarisation as K+ channels open causing rapid loss

Question 20

What is the advantage of the long refractory periods between APs in cardiomyocytes?

Answer 20

Cannot summate and tetanus